In 1981, Professor Akasaki of Osaka University in Japan developed successfully a light-emitting diode (LED) using a gallium-nitride (GaN) PN junction. The p-type GaN uses organometallic dicyclopenta magnesium as the source of the dopant magnesium. The grown thin film has to be illuminated by low-energy electron beams to activate the magnesium atoms, and then a p-type GaN thin film is produced. Besides, aluminum nitride is used as the buffer layer grown on a sapphire substrate. The brightness of their first LED using a GaN PN junction is 10 micro-candle power. Nichia Corporation didn't get involved in the research of GaN until 1989. They first innovated a two-stream metal-organic chemical vapor deposition (MOCVD) to grow high-quality GaN thin films. They used a GaN thin film grown at low temperature as the buffer layer, and adopted dicyclopenta magnesium as the source of the p-type dopants. The grown GaN thin film doped with magnesium was thermally treated, but not illuminated by low-energy electron beams as Professor Akasaki had done. In March of 1991, they developed their first LED with a homogeneous PN junction. Next, they also grew successfully indium gallium nitride (InGaN) thin films. In December of 1992, they successfully developed a high-power LED with heterogeneous junctions. Afterwards, they tried to grow LEDs with a single quantum-well structure and a multiple quantum-well structure using aluminum gallium nitride (AlGaN) or GaN as the confining layers. In 1994 and 1995, they published blue-green and green LEDs with 12 candle power in succession. In 1996, they announced to mass-produce blue-green LEDs.
The major reason for the LED industry to move forward is the success of buffer layer, p-type layer, InGaN active layer, and ohmic contact technologies for blue LEDs. Their structure evolved from homogeneous PN junction and heterogeneous junction (even dual heterogeneous junctions) to single and multiple quantum-well structures.
In comparison with gallium arsenide (GaAs), GaN-system materials, such as aluminum nitride (AlN)-GaN-indium nitride (InN) are semiconductor materials with wide bandgaps. Their bandgaps range from wider-bandgap AlN (bandgap=6.2 eV) to GaN (bandgap=3.4 eV), to narrower-bandgap InN (bandgap=2.0 eV). They can be adjusted to form ternary alloys. If the bandgaps of AlGaN and InGaN (the line connecting GaN and InN on their composition plot) are modulated, the wavelength can be modulated correspondingly. Thereby, LEDs or laser diodes with various colors can be fabricated.
The GaN material grown on a sapphire substrate, which is a hexagonal crystal, using epitaxy technology has a hexagonal structure. However, the lattice constants of the two are different. The lattice constant of the GaN grown epitaxially on a sapphire substrate is smaller than that of sapphire by approximately 16%. The lattice structure of other Ill-V compound semiconductors, such as GaAs, gallium phosphide (GaP), and indium phosphide (InP), is cubic crystal.
Furthermore, the fabrication technology of single-crystalline GaN substrate according to the prior art adopts MOCVD or hydride vapor phase epitaxy (HVPE) to form GaN bulk. Then laser ablation technology is used to separate the GaN bulk from the substrate and gives the single-crystalline GaN substrate. FIGS. 1A and 1B show schematic diagrams of a fabrication technology for a single-crystalline GaN substrate according to the prior art. First, a single-crystalline GaN bulk 102 is grown on a substrate 100 (as shown in FIG. 1A). Next, laser light is used to heat up the interface between the single-crystalline GaN bulk 102 and the substrate 100 to dissolve GaN into metal Ga and nitrogen molecules. Afterwards, the substrate 100 is heated up to the melting point of metal Ga, and then the single-crystalline GaN bulk 102 and the substrate 100 can be separated (as shown in FIG. 1B). The separated single-crystalline GaN bulk 102 is a single-crystalline GaN substrate. The drawback of this method is that laser is needed to heat up the interface between the single-crystalline GaN bulk and the substrate, which is time-consuming with a low yield.
FIGS. 2A and 2B show schematic diagrams of another fabrication technology for a single-crystalline GaN substrate according to another prior art. T. M. Katona et al. etched the surface of a silicon substrate 200 into a ridge structure 202. Then, GaN is grown on the surface of the silicon substrate 200 and form a single-crystalline GaN substrate 204. The drawback thereof is that GaN crystals 206 will grow at the bottom of the ridge structure 202 as well, which will produce large stress on the ridge structure 202 of the silicon substrate 200. Thereby, cracks might occur on the single-crystalline GaN substrate 204.
Accordingly, the present invention provides a method for fabricating a single-crystalline substrate containing gallium nitride, which can solve the time-consuming and low-yield problems using laser to heat up the interface between the GaN containing substrate and the host substrate. In addition, the present invention can avoid cracks on the single-crystalline GaN substrate by releasing the thermal stress between the GaN containing substrate and the host substrate.